This invention relates to protective coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a multilayer thermal barrier coating (TBC) with improved erosion and impact resistance as a result of alternating layers of the TBC containing alumina and/or chromia particles or precipitates.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components within the hot gas path of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to thermal damage and oxidation and hot corrosion attack, and may not retain adequate mechanical properties. For this reason, these components are often protected by a thermal barrier coating (TBC) system. TBC systems typically include an environmentally-protective bond coat and a thermal-insulating ceramic topcoat, typically referred to as the TBC. Bond coat materials widely used in TBC systems include overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), and diffusion coatings such as diffusion aluminides that contain aluminum intermetallics.
Ceramic materials and particularly binary yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. TBC""s employed in the highest temperature regions of gas turbine engines are often deposited by electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation of the TBC. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.). While YSZ coatings are widely employed in the art for their desirable thermal and adhesion characteristics, they are susceptible to thinning from mechanical damage during engine operation, particularly from impact and erosion by hard particles in the high velocity gas path. Impact damage and the resulting loss of TBC particularly occur along the leading edges of components such as turbine blades, while erosion is more prevalent on the concave surfaces of the blades. Both forms of mechanical damage not only shorten component life, but also lead to reduced engine performance and fuel efficiency. Though mechanical damage such as erosion can be addressed by increasing the thickness of the TBC, a significant drawback is an increase in thermal stresses within the coating, leading to a higher incidence of spallation. Consequently, other solutions are necessary to achieve an impact and erosion-resistant TBC with an acceptable thickness, preferably less than 250 micrometers.
Various attempts to produce more impact and erosion-resistant TBC""s for gas turbine engines have been directed to thermally treating the outer surface of the ceramic TBC material or providing an additional wear-resistant outer coating. Suggested materials for more wear-resistant outer coatings have included zircon (ZrSiO4), silica (SiO2), chromia (Cr2O3) and alumina (Al2O3) While various methods and apparatuses are capable of producing multilayered TBC by sequentially depositing layers of different materials, a difficulty has been a tradeoff between spallation resistance and thermal conductivity. Spallation resistance is generally reduced by the presence of abrupt compositional changes at the interfaces between layers. On the other hand, and as discussed in U.S. Pat. No. 5,792,521 to Wortman, if the interfaces between layers are characterized by localized compositional gradients containing mixtures of the different deposited materials, the interface offers a poorer barrier to thermal conduction as compared to a distinct compositional interface in which minimal intermixing exists. Though it is possible to employ low deposition rates and to physically reposition a component to be coated in order to ensure that discrete and homogeneous layers of minimal thickness are deposited, such a technique is impractical for mass-produced components such as turbine blades and nozzles of gas turbine engines.
Wortman discloses a method for depositing a multilayer TBC having alternating discrete and homogeneous layers, e.g., YSZ and alumina, by which compositional gradients are avoided. While the resulting TBC is characterized by lower thermal conductivity and enhanced resistance to spallation when subjected to erosion in the hostile thermal environment of a gas turbine engine, further improvements in TBC technology are desirable, particularly as TBC""s are employed to thermally insulate components intended for more demanding engine designs.
The present invention generally provides a thermal barrier coating (TBC) and method for forming the coating on a component intended for use in a hostile environment, such as the superalloy turbine, combustor and augmentor components of a gas turbine engine. The coating and method are particularly directed to a multilayer TBC that exhibits improved impact and erosion resistance as a result of being composed of discrete ceramic layers, each having a columnar grain structure. Improvements in impact and erosion resistance are notably evident with TBC""s formed of yttria-stabilized zirconia (YSZ).
The invention generally entails a multilayer thermal barrier coating having layers of precipitate-free YSZ alternating with layers of YSZ containing at least 3 volume percent up to about 50 volume percent of alumina and/or chromia particles and/or precipitates. In the form of particles and/or precipitates in these amounts, sufficient alumina and/or chromia is present to significantly increase the impact and wear resistance of the TBC while avoiding the presence of localized compositional gradients that would decrease the spallation resistance of the TBC. The particles/precipitates of alumina and/or chromia harden the YSZ and, therefore, the entire TBC more effectively than discrete layers of alumina or chromia. Contrary to Wortman, the invention also avoids the presence of abrupt compositional interfaces between the alternating layers of the TBC, for the purpose of reducing the incidence of spallation attributable to abrupt and consequently weak interfaces between the dissimilar materials (YSZ and alumina and/or chromia).
A preferred method for depositing the multilayer TBC of this invention is physical vapor deposition techniques, such as EBPVD, by which the TBC and the particles and/or precipitates are formed by evaporating multiple ingots, at least one of which is YSZ while a second contains alumina and/or chromia and optionally YSZ. In this method, the alumina and/or chromia content of the second ingot is intermittently evaporated during the deposition process to produce the precipitate-containing YSZ layers. Alternatively, the TBC can be deposited by evaporating a single ingot containing YSZ and regions of alumina and/or chromia, the latter of which is only intermittently evaporated along with YSZ during the deposition process to yield the precipitate-containing YSZ layers. Another alternative is to evaporate a single ingot of YSZ using a chemical vapor deposition (CVD)-assisted process in which a source of aluminum vapors is introduced into the coating chamber, causing oxidation of the aluminum and deposition of the resulting alumina vapors along with YSZ. The alternating layers of precipitate-free and precipitate-containing YSZ can be deposited by pulsing the CVD source. Another alternative method is to use an ion beam source of aluminum and/or chromium (cathodic arc source) that can be switched on intermittently while evaporating a YSZ ingot to create alternating layers of precipitate-free and precipitate-containing YSZ. With each of the alternative methods, the evaporation process is scalable to allow for the use of multiple YSZ ingots or YSZ sources, while also incorporating alumina and/or chromia in intermediate layers.
The resulting TBC is characterized by improved resistance to both erosion and impact as a result of the alumina and/or chromia particles and/or precipitates being present in sufficient amounts within a YSZ matrix, and without being present as discrete layers or in amounts that would embrittle the TBC. As a result, the present invention allows for the use of a relatively thin TBC because the TBC is not required to be deposited to a thickness sufficient to allow for erosion rates typically seen with conventional YSZ TBC. The net benefit is improved component life, engine performance and fuel efficiency.
Other objects and advantages of this invention will be better appreciated from the following detailed description.